Phenotypic characterisation of a Neospora caninum temperature-sensitive strain in normal and immunodeficient mice

The in vivo persistence, immunogenicity and pathogenicity of a recently described temperature-sensitive (ts) strain from Neospora caninum, NCts-8, was investigated in normal and immunodeficient mice. Groups of BALB/c and SCID/Bg mice were infected s.c. with 5 x 10(6) wild-type NC-1, control NCts-8 (pass 0) or NCts-8 tachyzoites prepared at four in vitro passage levels (pass 7, 13, 21 and 28). For persistence and immunogenicity studies, BALB/c mice were bled and sacrificed at 4, 6 or 8 weeks p.i. Sera were analysed by IFAT and brain tissues examined for lesions by histology and tested for parasite presence by PCR. For pathogenicity studies, SCID/Bg mice were monitored by clinical signs and survival time. Results from parasite persistence experiments demonstrated microscopic lesions and PCR positive brain tissues in NC-1 infected mice. In contrast, brain tissues from NCts8-infected groups were consistently negative by histology and PCR. Based on IFAT titres, all parasite strains were immunogenic, although parasite-specific IgG levels were lower in the NCts-8 infected groups. Results from pathogenicity studies in SCID/Bg mice demonstrated a significantly (P < 0.0001) longer mean survival time in NCts-8 vs NC-1 infected groups. In addition, there was no significant difference in mean survival time between control NCts-8 and experimental passage NCts-8 infected mice. Collectively, these studies demonstrate that the NCts-8 strain maintains a stable phenotype following multiple passages in vitro, and possesses an attenuated, shorter persistence phenotype in vivo compared with the parental wild-type NC-1.